1. Field of the Invention
Provided are open metal carbonyl clusters and a method for synthesizing the open metal carbonyl clusters. More specifically, provided is the synthesis of open Ir4 carbonyl clusters carrying phosphine ligands by selective oxidation using an oxidant. The open metal clusters are useful as reagents in a variety of chemical transformations.
2. Description of the Related Art
Increasing catalytic activity, particularly for hydrogenation catalysts, is always a valued goal. There are reports of oxidative activation of catalyst sites for homogeneous cationic complexes used in hydrosilylation. See, Organosilicon Chemistry. Part 24. Homogeneous Rhodium-catalysed Hydrosilation of Alkenes and Alkynes: The Role of Oxygen or Hydroperoxides by Parish et al. in J C S Dalton 1980, 308-313) and hydrogenation reactions (Pentamethylcylcopentadienyl-Rhodium and -Iridium Complexes Part 35Hydrogenation Catalysts Based on [(RhC5Me5)2(OH)3) And The Border Between Homogeneous and Heterogeneous Systems by Maitlis et al. in J Mol. Cat. 1982, 15,337-347. These studies are preceded by reports of increased hydrogenation activity after oxygen treatment; for instance, a 100-fold increase in hydrogenation activity of maleic acid is observed upon treating the homogeneous trans-IrX(CO)(PPh3)2, where X═Cl, Br complex with small amounts of oxygen (Kinetic study of iridium (I) complexes as homogeneous hydrogenation catalysts by James and Memon in Can J. Chem. 1968, 46:217-223). Both the Parish et al. and Maitlis et al. manuscript attribute the role of oxygen treatment as one that removes ligands (e.g., oxidizes triphenylphosphine to triphenylphosphine oxide), thereby creating a coordinatively unsaturated center that is catalytically active. The Maitlis et al. article articulates how such species are unstable and readily aggregate into larger particles in general.
Open metal carbonyl clusters are known, as are various syntheses therefore. For example, “Stabilization of coordinatively unsaturated Ir4 clusters with bulky ligands: a comparative study of chemical and mechanical effects”, by Alexander Okrut et al., Dalton Trans., (2012) 41:2019, describes the synthesis of a metal carbonyl cluster that has open sites. The cluster is synthesized thermally and stabilized with calixarenes.
Trimethylamine oxide is used in the synthesis of ligand-substituted clusters, as shown in “Synthesis and structural characterization of the isomers of Rh6 (CO)14L2 clusters (L=NCMe, P4, P(OPh)3), X-ray crystal structure of trans-Rh6(CO)14 {P(OPh3)}2”, by Tunik et al. Journal of Organometallic Chemistry, (1994) 479:59-72.
The use of a brominating agent in the synthesis of ligand-substituted clusters is described in “New Tetrahedral Cluster Compounds of Iridium. Synthesis of the Anions [IR4(CO)11X]− (X═Cl, Br, I, Cn, SCN) and X-ray structure of [PPh4] [Ir4(CO)11Br]”, Chini et al., Journal of Organometallic Chemistry, (1978) 152:C35-C38. See also “Chemistry of Tetrairidium Carbonyl Clusters. Part 1. Synthesis, Chemical Characterization, and Nuclear Magnetic Resonance Study of Mono- and Di-substituted Phosphine Derivatives. X-Ray Crystal Structure Determination of the Diaxial Isomer of [Ir4(CO)7(μ-CO)3(Me2PCH2CH2PMe2)]”, Ros et al., J. Chem. Soc., Dalton Trans. (1986); and “Patterned metal polyhedra using calixarenes as organizational scaffolds: Ir4-based cluster assemblies”, de Silva et al., Dalton Trans., (2010) 39:2194-2197.
The synthesis of an open gold cluster has open sites and is stabilized with calixarene ligands is described in “A bioinspired approach for controlling accessibility in calyx [4]arene-bound metal cluster catalysts”, by de Silva et al., Nature Chemistry, (2010) 2:1062-1068.
The synthesis of an open gold nanoparticles that has open sites and is stabilized with calixarene ligands is described in “Synthesis and Characterization of Accessible Metal Surfaces in Calixarene-Bound Gold Nanoparticles”, by Jeong-Myeong Ita et al., Langmuir, (2009) 25(18):10548-10553.
An objective of the present invention is to provide a simple and efficient synthesis for an open metal carbonyl cluster, which open cluster is useful as a catalyst, is stable, and demonstrates improved catalytic activity.